1. Field of the Invention
The present invention generally relates to electronic data transmission systems, and more specifically to a method of suppressing crosstalk in wide, high-speed buses, particularly buses such as those used in and between various components of computer systems, and further to novel transmission circuits which implement the crosstalk suppression.
2. Description of Related Art
Modern electronic circuits use many different types of logic components (processing units) to carry out numerous functions. These circuits require a multitude of conductive pathways to provide communications or connectivity between the logic components. Many components require a group or set of conductors (wires), referred to as a bus, which interconnect a plurality of related output lines of one device to a respective plurality of input lines of another device, often in a parallel fashion. Buses are most advantageously used to interconnect three or more devices, and allow simultaneous or sequential access to information conveyed on the bus. These buses may be external, e.g., laid out on a printed circuit board, and interconnecting two or more devices which are separately packaged. They may also be internal, interconnecting two or more devices which are fabricated in a single package, such as an integrated circuit (IC). In the latter implementation, a bus line may be so small (narrow) as to be hardly visible to the naked eye.
Buses are used to interconnect devices for a wide variety of applications, including communications between complex computer components such as microprocessors, application specific integrated circuits (ASICs), peripheral devices, random-access memory, etc. Operational demands on high-speed buses for computer systems have especially increased with the advent of larger bus widths. For example, early computer processors used 8-bit operation (8-bit registers and execution units) and consequently required an 8-line bus. Just in the past few years, however, data bus requirements for IC chips and microprocessors have gone from 16, to 32, to 64, and to 128 lines. The most recent advances are directed to 256-line buses.
With the increasing complexity of microprocessor circuits and their associated bus requirements, and further considering the ongoing miniaturization of integrated circuits, the physical constraints on data bus widths present several difficulties. One problem is electromagnetic interference (i.e., coupling effects) between parallel conductors. Each conductor exhibits a capacitance with respect to its immediately adjacent line(s), as well as with respect to more distant lines. Excessive interference between neighboring conductors can result in "crosstalk," wherein a data signal from one or more lines interferes with the signal on a nearby line, i.e., changes the voltage on the line sufficiently to cause a bit error. Crosstalk problems can also arise with orthogonal lines, as well as bidirectional and broadcast buses. This problem is particularly exacerbated with high-speed buses (e.g., those operating at frequencies around 100 MHz or more, and especially above 1 GHz), and can result in undetectable, sporadic errors. Crosstalk can additionally occur between lines from adjacent buses, e.g., in a multilayer circuit.
Trends accordingly indicate that the data bus will become a data processing bottleneck in the near future, by which it is meant that the IC or microprocessor will be able to process, present and receive parallel data faster than the available data buses can reliably transmit the data. There are other methods of reducing crosstalk, which would allow increased bus speed, but all of these have significant limitations or disadvantages. For example, the physical spacing between immediately adjacent bus lines can be increased, to lower the mutual capacitances, but this approach has deleterious consequences on miniaturization. It would, therefore, be desirable to devise an improved method of suppressing crosstalk in a data transmission bus, so as to allow higher operation speeds for the bus. It would be further advantageous if the method of improved crosstalk suppression were able to maintain a low interconnection delay.